Capacitive discharge ignition system for internal combustion engines

ABSTRACT

The various circuits use the primary coil and breaker points of conventional inductive ignition systems (magneto or battery) as the charging and timing elements in a capacitive discharge conversion system. When the breaker points open, the stored energy is transferred from the coil to capacitor C1 which is discharged through a low impedance transformer to obtain a rapid voltage rise time to obtain improved performance of the spark plug connected to the secondary of the transformer. In one circuit the capacitor is discharged near peak voltage by triggering a thyristor in response to the inital discharge of the capacitor from its peak charge. In other circuits the thyristor is triggered to discharge the capacitor when the charge on another capacitor in an R-C circuit shunting capacitor C1 reaches the breakdown voltage of a trigger diode, the R-C time constant being selected to enable the charge capacitor C1 to reach its maximum charge voltage.

United States. Patent [1 1 Cavil [451 Mar. 18, 1975 i 1 CAPACITIVEDISCHARGE IGNITION SYSTEM FOR INTERNAL COMBUSTION ENGINES [75] Inventor:David T. Cavil, Menomonee Falls,

[21] App]. No.: 330,443

[52] U.S. Cl. 123/148 E, 315/209 [51] Int. Cl. F02p 1/00 [58] Field ofSearch 123/148 E, 149; 310/153,

[56] References Cited UNITED STATES PATENTS 3,367,314 9/1965 Hirosawa eta1. 123/148 Primary ExaminerManuel A. Antonakas Assistant Examiner-J. W.Cranson Attorney. Agent, or Firm-Michael, Best & Friedrich [57] ABSTRACTThe various circuits use the primary coil and breaker points ofconventional inductive ignition systems (magneto or battery) as' thecharging and timing elements in a capacitive discharge conversionsystem. When the breaker points open, the stored energy is transferredfrom the coil to capacitor C which is discharged through a low impedancetransformer to obtain a rapid voltage rise time to obtain improvedperformance of the spark plug connected to the secon-' dary of thetransformer. In one circuit the capacitor is discharged near peakvoltage by triggering a thyristor in response to the inital discharge ofthe capacitor from its peak charge. In other circuits the thyristor istriggered to discharge the capacitor when the charge on anothercapacitor in an R-C circuit shunting capacitor C, reaches the breakdownvoltage of a trigger diode, the R-C time constant being selected toenable the charge capacitor C to reach its maximum charge voltage.

PATENIEUMAR 81975 SHEET 1 0f 2 \1 Iii PATENYEB HA8 1 8l975 suinep zCAPACITIVE DISCHARGE IGNITION SYSTEM FOR INTERNAL COMBUSTION ENGINESBACKGROUND OF THE INVENTION Inductive ignition systems of the type usedon snowmobile engines and outboard motors have a tendency to foul sparkplugs at low speeds, at which speeds the secondary voltage rise time istoo low. In the case of snowmobile engines, the problem is morepronounced and no plugs offer an operating heat range suitable for bothhigh power output and idle speed as well. The-reasons for this arerelated to permissible current across the breaker points and to theinductive limitations inherent in these systems. Proper selection ofcoil design can keep the switching current across the breaker pointswithin reasonable limits but practical current'interrupting capacity ofbreaker points imposes a limitation on the available energy storagesince a relatively large amount of inductance is thenrequired andresults in the secondary voltage rise time being too slow for use withcolder spark plugs because of their tendency to foul at low speeds.

Capacitive discharge (CD) systems use high frequency, .low impedanceignition coils which provide very rapid rise time on the secondaryacross the plugs improving'cold starting, operating heat range andability to tire fouled plugs. Obviously, CD ignition is attractive insnowmobile engines and outboard motors but has been incorporated inoriginal equipment only on a limited basis due to cost. Conversion CD(and transistor) systems are available for automotive engines but havenot been compatible with magneto systems. Most automotive CD conversionsystems use the breaker points. even though such use retains theproblems of point wear, etc. albeit to a reduced extent since thecurrent is reduced to unacceptable levels. for the uses contemplatedherein since snowmobile engines and outboard motors generally require atleast 3 amps on the points to operate properly in the oily conditions ofthese engines. Ideally, CD ignition is breakerless but the cost of suchsystems is unacceptable for conversion or for many original equipmentsituations.

SUMMARY OF THE INVENTION The invention provides a capacitive dischargeignition system for internal combustion engines, which system comprisesa source of electrical potential, a charge capacitor in circuit with thesource of electrical potential to be charged thereby, breaker pointsconnected with the source of electrical potential and operative whenopened to cause the source of electrical potential to charge the chargecapacitor, an ignition primary winding in circuit with the chargecapacitor, switch means in circuit with the primary winding and thecharge capacitor and including a control terminal operative to turn theswitch means on so as to discharge the charge capacitor in response to apredetermined trigger potential, and a circuit shunting the chargecapacitor and including a resistance and a second capacitor, whereby thecharge buildup on the second capacitor lags the buildup on the chargecapacitor by the time constant of the shunt circuit the potential on thesecond capacitor being applied to the control terminal and the values ofthe resistance and of the second capacitor being selected so that thetrigger potential is applied to the control terminal at approximatelythe time of maximum charge on the charge capacitor.

In one embodiment in accordance with the invention the source ofelectrical energy comprises means for storing energy in a coil.

In one embodiment in accordance with the invention the values of theresistance and of the second capacitor are selected so that thebreakdown potential of the control terminal occurs when the potential onthe second capacitor is less than the potential on the charge capacitor.

In one embodiment in accordance with the invention the switch means is athyristor including a gate responsive to the potential on the secondcapacitor.

FIG. 1 is a schematic representation of a capacitive dischargeconversion of a magneto ignition system.

FIG. 2 is the equivalent charging circuit of FIG. I.

FIG. 3 is the equivalent triggering circuit of FIG. 1.

FIG. 4 is the equivalent discharge circuit of FIG. I.

FIG. 5 is a schematic representation of a capacitive dischargeconversion of a battery ignition system.

FIG. 6 is a schematic diagram of a phase control type of triggeringsystem.

FIG. 7 is a schematic representation of a twin cylinder, single coilsystem utilizing two rotating magnets.

FIG. 8 is a schematic representation of a twin cylinder capacitivedischarge system utilizing one rotating magnet (for graphic purposes,shown as two) and two coils.

DESCRIPTION OF PREFERRED EMBODIMENT FIG. I is a schematic representationof a capacitive discharge conversion of a snowmobile magneto system.Here the flywheel (or other rotating part) is provided with a magnet 10which rotates in proximity to the magneto primary 12 to store energy inthe primary. The usual breaker points 14 are across the primary as isthe usual capacitor C In the standard magneto arrangement the magnetosecondary would have current induced therein and be connected directlyto the spark plug(s). In a low tension magneto such as usually employedon snowmobiles, the leads 16,18 would be connected to a separateignition coil having a primary and secondary with the secondary beingconnected to the spark plug(s). The system about to be described can beutilized with either the standard magneto or low tension magneto. Inconnection with the low tension magneto, the entire ignition coil wouldbe removed and replaced with the parts about to be described. In astandard magneto system, the secondary is discarded.

In a snowmobile engine there is provision for spark advance and as thisoccurs there is a reversal of polarity in the region of, say, 600 rpm.Due to this reversal of polarity, the rectifier bridge 20 is provided tomaintain constant polarity on the system. The principle of a rectifierbridge is well understood. Suffice it to say that it operates tomaintain junction 22 positive and junction 24 negative. When the pointsopen, the energy stored in the magneto primary 12 will charge capacitorC through the bridge, junction 22 and line 26. Lead 26 is also connectedto the primaries of the pulse transformers 28 which are then connectedto anode 30 of the silicon controlled rectifier SCR. The cathode of theSCR is connected to the other side ofC through leads 32,34 and is alsoconnected to the anode of diode D the cathode of which is connected tothe negative side of the bridge 20. The gate of the SCR is controlled bythe Zener diode D Zener diode D in the gate circuit serves todesensitize the circuit to transients induced by point arcing andresistor R shunting capacitor C, is for the same prupose. Diode D andresistor R are removed for the sake of clarity in considering theequivalent charging circuit shown in FIG. 2. In this equivalent circuitthe rectifier bridge is not shown as such but diode D represents theblocking action of the bridge and diode D 2 represents the reverseshunting action of the bridge. When the breaker points open, chargecapacitor C, and the capacitor across the breaker points (the normalcapacitor associated with the magneto and which may be omitted ifdesired) are charged exponentially from the primary coil along the pathindicated by the dotted arrows in FIG. 2.

After the charge capacitor C, has been charged to peak voltage, itbegins to discharge along the path indicated by the dotted arrows inFIG. 3 which illustrates the equivalent triggering circuit. This resultsin the gate of the SCR going positive with respect to the cathode torender the SCR conductive and discharge C, (and C into the pulsetransformers as indicated in FIG. 4. Diode D (the-rectifier bridge)clamps the capacitor in the reverse direction and provides afree-wheeling path for the transformer primaries. Two transformers areshown and this is typical of snowmobile engines having opposed cylindersfired in unison.

FIG. 5 shows the conversion of a battery system to capacitive discharge.Again, only the primary coil 36 is utilized, the secondary beingdiscarded in the conversion. When the breaker points 38 are closed, thecoil is energized from battery 40 through ballast 42. Capacitor Cnormally provided, may be used or not, as desired in the conversion.When the breaker points open, the charge capacitor C, is charged throughline 44 which also leads to the primary of the ignition transformer 46.The other side of the primary is connected to the anode of the SCR, thecathode of which is connected to junction 48 and diode D, leading toline 50. Resistance R is connected between line 50 and the gate of theSCR. This system is also automatically triggered upon the capacitor C,reaching maximum charge. As the charge starts to leak through theprimary coil 36, line 50 and resistance R, the gate of the SCR goespositive, fires the SCR, and discharges capacitor C, through the primaryof the ignition transformer 46. Diode D, provides a free-wheeling pathfor the transformer and clamps the capacitor. The source of energy forthe primary coil 36 in this case is a battery. In the prior circuit itwas a magneto. The source is not the important part of this invention.The use of the primary coil as an energy storage device and the use ofthe breaker points to control the charging and timing of a capacitivedischarge system is the crux of the invention.

Another method of timing the discharge of the charge capacitor isillustrated in FIG. 6. Here again there is a rotating magnet 10, amagneto primary 12 with the breaker points 14 and the optional(original) magneto capacitor C across the primary. In this arrangementthe charge capacitor C, is shown across the lines 16,18 although it willbe understood that if polarity reversal is a factor, then a rectifierbridge would be interposed. When the breaker points open in thiscircuit. the capacitor C, is charged. The capacitor C is also chargedthrough resitance R. The charge on C is applied to the Zener diode D,(which could be a diac or any other trigger diode) and when the Zenerdiode breaks down, the voltage will be applied to the gate of the SCR tofire the SCR and discharge capacitor C,

through the primary of the ignition transformer 52, and the thyristor(SCR) to the grounded side of the circuit. Diode D again functions toprovide the free-wheeling path for the primary of the ignitiontransformer and to clamp C, in the reverse direction. In this circuit itwill be noted that the Zener diode D, breaks down when the voltage on Creaches the breakdown value. The time required to reach this breakdownvalue is determined by the time constant of R and C The values of R andC are adjusted (selected) so the voltage across C reaches the breakdownvoltage of the Zener diode D, at a time sufficiently delayed in time toenable the charge capacitor C, to reach its maximum charge voltage. Theresistor-capacitor combination R,C,, which shunts the charge capacitoris a phase control type circuit. This type of phase control is used inAC. power control but, to my knowledge, has not been used in the presenttype of environment.

The phase control type triggering system shown in FIG. 6 can be madebi-directional as in FIG. 7. In this instance the flywheel is providedwith two rotating magnets having opposite polarity. The points 14 areoperated by a cam which opens the points each time a magnet passes theprimary coil 12. Because the magnetization is opposite, the primarycurrent in coil 12 will alternately be positive and negative and thepoints will open on alternately positive and negative current peaks.This will cause capacitor C, to be charged alternately positive andnegative with respect to ground. The original and optional capacitor C,will also be charged positive and negative with respect to ground. Inthis circuit the trigger device D, is a diac which is a bi-directionaltrigger diode. It will be conductive when the voltage across it reachesthe breakdown voltage regardless of polarity. One anode of the diac D,is connected to the capacitor C and the other anode is connected to thegate of the thyristor which in this case is a triac. Anode 1 of thetriac is connected to ground and anode 2 of the triac is connected tojunction 54. The primary coil 56 of ignition transformer 58 is connectedbetween line 16 and junction 60.

The primary 62 of transformer 64 is connected between junction andjunction 54. When a positive voltage across C reaches the breakdownvoltage of the diac D,, the diac breaks down to trigger the triac, andC, is discharged through diode D leading from line 16 to junction 66 tojunction 60 and then through coil 62 of ignition transformer No. 2 whichfires the spark plug connected to the secondary of the transformer.Diode D, acts under these conditions to clamp the capacitor and providefree wheeling for transformer No. 2. N616 the triac acts the same as theSCR in the previous circuit. When C, is charged in the negativedirection, C, is also charged in the negative direction and when thevoltage across C reaches the breakdown voltage of the diac (in theopposite direction to that just described), the diac conducts and firesthe triac gate (the gate becomes negative with respect to its anode No.1). This discharges C, through the triac and diode D and the primary 56of ignition coil No. 1 while diode D acts to clamp the capacitor. Thusthis system can be utilized in conjunction with converting an alternatefiring twin cylinder ignition system.

If it is desired to reduce the rotating mass, a single rotating magnetcan be used in conjunction with two magneto primary coils and twobreaker point sets as in dicated in FIG. 8. While at first blush thisdrawing appears to have two rotating magnets, bear in mind but onemagnet is used and is denoted 68. The two coils are denoted 70,72. Thebreaker point sets are denoted 74,76. The two original capacitors C ,C.,may be optionally left in the system. One breaker point set will beclosed when the other is opened so the coils 70,72 are dischargedalternately. When points 74 open, assuming that line 78 is charged inthe positive sense, a positive charge is put across charge capacitor C,and across capacitor C The junction 80 of the diac is now going negativeand when the breakdown voltage of the diac is reached, the gate of thetriac will fire the triac to allow the charge capacitor to dischargethrough diode D the triac, and the primary coil 82 of the ignitiontransformer 84 for cylinder No. 1 while diode D clamps the capacitor.When breaker points 76 open, the line 86 goes positive to chargecapacitor C in the opposite direction and also to charge C in theopposite direction, thus having junction 80 going positive and,

when the breakdown voltage of the diac is reached, it

triggers thetriac which tires to allow the capacitor C, to dischargethrough diode D the triac, and the primary coil 88 of the ignitiontransformer 90 for cylinder No. 2 while diode D now functions to clampthe capacitor C.

The time constant factors described relative to FIG. 6 apply inconjunction with the circuit of FIG. 8. The phase control typetriggering system, whether it be the single cylinder type shown in FIG.6 or the twin cylinder type shown in FIGS. 7 and 8, is not as precise asthe system first described insofar as triggering at the peak charge isconcerned. At low engine speeds, when the charge voltage is slightlylower, thetime delay is longer and this may allow the charge capacitorto partially discharge back through the primary coil. By way ofcompromise, the trigger point can peak at low speeds while accepting aslight loss of charge voltage at higher speeds due to triggering beforepeak charge voltage is reached. This triggering system is'also moretemperature sensitive than the first system.

This invention has primary application to conversion of snowmobileengines and as original equipment in snowmobiles where costs arecritical. Application to outboard motors is feasible but not asattractive since the cost relative to present breakerless capacitivedischarge systems for outboard motors offers no significant advantageand does have some disadvantages due to retention of breaker points. TheCD systems described herein do offer the possibility of application toexisting outboard motors, however, and would offer an upgrading ofperformance.

It will be noted the breaker points control both the charging of thecharge capacitor and the timing in that the time of discharge of thecapacitor is ultimately related in time to the opening of the points.Systems have been proposed in the past where the points controlcharging, and timing is controlled by voltage responsive means whichmerely look for a minimum voltage. Other systems have the breakerscontrol timing (by controlling a trigger) while other means controlcharging. The present system using energy stored in an induction coilcan control both charging and timing with the points.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A capacitive discharge ignition system for internal combustionengines, said system comprising a coil, means for storing energy in saidcoil, a charge capacitor in circuit with said coil to be chargedthereby, breaker points connected across said coil and operative whenopened to cause the energy stored in said coil to charge said capacitor,an ignition primary winding in circuit with said charge capacitor,switch means in circuit with said primary winding and said chargecapacitor and including a control terminal operative to turn said switchmeans on so as to discharge said charge capacitor in response to apredetermined trigger voltage, a circuit shunting said charge capacitorand including a resistance and a second capacitor, whereby the chargebuildup on said second capacitor lags the buildup on said chargecapacitor by the time constant of said shunt circuit, the voltage onsaid second capacitor being ap plied to said control terminal and thevalues of said resistance and of said second capacitor being selected sothat said trigger voltage is applied to said control terminal atapproximately the time of maximum charge on said charge capacitor.

2. A capacitive discharge ignition system according to claim 1 in whichsaid switch means is a thyristor including a gate responsive to voltageon said second capacitor.

3. A capacitive discharge ignition system in accordance with claim 1wherein the values of said resistance and of said second capacitor beingselected so that the breakdown voltage of said control terminal occurswhen the voltage on said second capacitor is less than the voltage onsaid charge capacitor.

4. A capacitive discharge ignition system for internal combustionengines, said system comprising a source of electrical potential, acharge capacitor in circuit with said source of electrical potential tobe charged thereby, breaker points connected with said source ofelectrical potential and operative when opened to cause said source ofelectrical potential to charge said charge capacitor, an ignitionprimary winding in circuit with said charge capacitor, switch means incircuit with said primary winding and said charge capacitor andincluding a control terminal operative to turn said switch means on soas to discharge said charge capacitor in response to a predeterminedtrigger potential, a circuit shunting said charge capacitor andincluding a resistance and a second capacitor, whereby the chargebuildup on said second capacitor lags the buildup on said chargecapacitor by the time constant of said shunt circuit, the potential onsaid second capacitor being applied to said control terminal and thevalues of said resistance and of said second capacitor being selected sothat said trigger potential is applied to said control terminal atapproximately the time of maximum charge on said charge capacitor.

5. A capacitive discharge ignition system in accordance with claim 4wherein the values of said resistance and of said second capacitor beingselected so that the breakdown potential of said control terminal occurswhen the potential on said second capacitor is less tha the potential onsaid charge capacitor.

6.A capacitive discharge ignition system according to claim 4 in whichsaid switch means is a thyristor including a gate responsive to thepotential on said sec-

1. A capacitive discharge ignition system for internal combustionengines, said system comprising a coil, means for storing energy in saidcoil, a charge capacitor in circuit with said coil to be chargedthereby, breaker points connected across said coil and operative whenopened to cause the energy stored in said coil to charge said capacitor,an ignition primary winding in circuit with said charge capacitor,switch means in circuit with said primary winding and said chargecapacitor and including a control terminal operative to turn said switchmeans on so as to discHarge said charge capacitor in response to apredetermined trigger voltage, a circuit shunting said charge capacitorand including a resistance and a second capacitor, whereby the chargebuildup on said second capacitor lags the buildup on said chargecapacitor by the time constant of said shunt circuit, the voltage onsaid second capacitor being applied to said control terminal and thevalues of said resistance and of said second capacitor being selected sothat said trigger voltage is applied to said control terminal atapproximately the time of maximum charge on said charge capacitor.
 2. Acapacitive discharge ignition system according to claim 1 in which saidswitch means is a thyristor including a gate responsive to voltage onsaid second capacitor.
 3. A capacitive discharge ignition system inaccordance with claim 1 wherein the values of said resistance and ofsaid second capacitor being selected so that the breakdown voltage ofsaid control terminal occurs when the voltage on said second capacitoris less than the voltage on said charge capacitor.
 4. A capacitivedischarge ignition system for internal combustion engines, said systemcomprising a source of electrical potential, a charge capacitor incircuit with said source of electrical potential to be charged thereby,breaker points connected with said source of electrical potential andoperative when opened to cause said source of electrical potential tocharge said charge capacitor, an ignition primary winding in circuitwith said charge capacitor, switch means in circuit with said primarywinding and said charge capacitor and including a control terminaloperative to turn said switch means on so as to discharge said chargecapacitor in response to a predetermined trigger potential, a circuitshunting said charge capacitor and including a resistance and a secondcapacitor, whereby the charge buildup on said second capacitor lags thebuildup on said charge capacitor by the time constant of said shuntcircuit, the potential on said second capacitor being applied to saidcontrol terminal and the values of said resistance and of said secondcapacitor being selected so that said trigger potential is applied tosaid control terminal at approximately the time of maximum charge onsaid charge capacitor.
 5. A capacitive discharge ignition system inaccordance with claim 4 wherein the values of said resistance and ofsaid second capacitor being selected so that the breakdown potential ofsaid control terminal occurs when the potential on said second capacitoris less than the potential on said charge capacitor.
 6. A capacitivedischarge ignition system according to claim 4 in which said switchmeans is a thyristor including a gate responsive to the potential onsaid second capacitor.